The present invention relates to an infrared detector and, in particular, to an infrared detector applied to an infrared focal plane array.
Here, Gt is the thermal conductance of the infrared detector, and according to the formula (1), it is effective for increasing the sensitivity Res to reduce this thermal conductance Gt. Usually, for that reason, the infrared detector is operated in a vacuum where the thermal conductivity of air is negligible. Further, the smaller the cross section of the support beam 105 is and the longer the support beam 105 is, the smaller the thermal conductivity becomes, which results in an improvement in the sensitivity of the infrared detector.
A conventional infrared detector, as shown in FIG. 22, is constituted by a semiconductor substrate 103 having a concave portion 104 on the surface thereof, a detection part 101 supported in a hollow state by a support beam 105 above the concave portion, and an absorption part 108 for effectively absorbing incoming infrared rays. The detection part 101 has a plurality of PN junction diodes 102 which are connected in series by means of a metal wire 106 so as to improve the sensitivity of the infrared detector. Further, a metal wire 107 is arranged for connecting the detection part 101 to a circuit made in the semiconductor substrate 103 through the support beam 105. The reason that the detection part 101 is supported in a hollow state apart from the semiconductor substrate 103 is to improve thermal insulation for the purpose of effectively increasing the temperature of the detection part by the incoming infrared rays. Still further, the detection part 101, the support beam 105 and a signal wire 109 are covered with an insulating material such as silicon oxide or the like.
Further, the plurality of PN junction diodes 102 are connected in series by the metal wire 106 in order to improve the sensitivity of the infrared detector. These PN junction diodes 102 are connected to a metal wire 107 embedded in the support beam 105 and the metal wire 107 is further connected to a signal wire 109 for transmitting a signal to the circuit. The performance of the infrared detector is determined by the ratio of sensitivity to noises. If there is a crystal defect or a crystal interface where crystals are in contact with each other, noises are produced. Therefore, a single crystal silicon thin film is most suitable for a silicon film in which a PN junction diode is formed. A SOI (silicon on insulator) substrate in which a silicon thin film is formed on a silicon substrate via an insulating film can be used as a semiconductor substrate and is suitable for reducing noises.
Here, the sensitivity of the infrared detector using the PN junction diodes will be described in the following. Since the sensitivity of the infrared detector Res (V/K) is proportional to the thermal coefficient of a diode dvf/dT (V/K) and is inversely proportional to the thermal conductance of the infrared detector Gt (W/K), the sensitivity of the infrared detector Res (V/K) can be expressed by the following formula (1)
Resxe2x88x9d(dvf/dT)/Gtxe2x80x83xe2x80x83(1)
Here, Gt is the thermal conductance of the infrared detector, and according to the formula (1), it is effective for increasing the sensitivity Res to reduce this thermal conductance Gt. Usually, for that reason, the infrared detector is operated in a vacuum where the thermal conductivity of air is negligible. Further, the smaller the cross section of the support beam 105 is and the longer the support beam 105 is, the smaller the thermal conductivity becomes, which results in an improvement in the sensitivity of the infrared detector.
Further, in the case where a thermal image is detected by the use of an infrared focal plane array constituted by such an infrared detector and is displayed on a CRT screen, if the speed of the thermal response of the infrared focal plane array is slow, an after image is produced on the CRT screen to degrade an image quality.
The thermal response characteristics of the infrared focal plane array will be described in the following. The thermal response characteristics of the infrared focal plane array is usually estimated by the thermal time constant xcfx84 designated by the following formula (2).
xcfx84=Ct/Gtxe2x80x83xe2x80x83(2)
Here, Ct is a heat capacity which is determined by the volume of a detection part (corresponding to the sum of the volume of the detection part 101 and the volume of the absorption part 108 in the example of FIG. 22) and the specific heat and the density of a material used for the detection part. As the thermal time constant xcfx84 becomes smaller, the response of the infrared focal plane array becomes better. It is necessary to increase conductance in order to reduce the thermal time constant xcfx84, so the required value of the conductance varies depending on a system to which the infrared detector is applied. In an NTSC (National Television System Committee) format, if the thermal time constant xcfx84 exceeds 30 msec, which is a period of an image display in a CRT, an after image appears.
As described above, in the conventional infrared detector, if the thermal conductance is decreased so as to improve sensitivity, reversely, the thermal time constant increases. Therefore, it is difficult to improve both characteristics at the same time.
An infrared detector according to the present invention is invented so as to solve the above mentioned problem and includes a semiconductor substrate provided with a single crystal silicon thin film arranged and held in a hollow state at a predetermined distance above the semiconductor substrate, a plurality of thermoelectric changing means which are embedded in the single crystal silicon thin film and able to change heat energy generated by an infrared ray irradiated to the single crystal silicon thin film to an electric signal, a first connecting layer which are embedded in the single crystal silicon thin film and electrically connecting the plurality of thermoelectric changing means to each other and a second connecting layer for transmitting the electric signal outputted from the thermoelectric changing means to wire formed in the semiconductor substrate. And further, at least one of said first and second connecting layers is constructed by a silicon compound.
Further, according to the present invention, the plurality of thermoelectric changing means may be connected in series by the first connecting layer.
Further, the single crystal silicon thin film may be arranged and held in a hollow state at a predetermined distance above the semiconductor substrate by a support beam.
In such an infrared detector, the second connecting layer may be embedded in the support beam.
In such an infrared detector, the second connecting layer may have a thickness different from a thickness of the single crystal silicon thin film.
In such an infrared detector, the second connecting layer may be formed of a material different from a silicon compound constituting the first connecting layer.
Further, such an infrared detector may include an infrared ray receiving part arranged in front of the single crystal silicon thin film and connected to the single crystal silicon thin film by a support column.
In such an infrared detector, the thermoelectric changing means may be formed of a junction diode, a bipolar transistor, a junction field effect transistor, a MOS transistor, or a Schottky barrier diode, or a combination of them.